Sedimentary Facies Of The Middle Part Of Shennongjia Group

The study area is Shennongjia region, located in northern margin of the Yangtze plate, this paper focuses on the middle part of Shengnongjia group in middle Proterozoic, namely Luanshigou formation, Dawokeng formation, Kuangshishan formation and Taizi formation. Based on predecessors’research,and the field work, including the regional geological work and 1:500 profile measured,it can have three Main conclusions:(1) There are three types of rocks, namely carbonate rocks, clastic rocks and mixed sedimentary rock. Dolomite particles, crystalline dolomite, stromatolite dolomite and siliceous dolomite etc.,which are carbonate rocks, mainly developed in Luanshigou formation, Dawokeng formation, Kuangshishan formation, generally indicated shallow water environment of carbonate platform;Quartz sandstone, siltstone, mudstone and shale etc.,which are clastic rocks, mainly developed in the upper part of Taizi formation; silty dolomite, muddy dolomite, dolomitic siltstone, dolomitic mudstone etc., which are mixed sedimentary rocks, mainly developed in the upper part of Luanshigou formation and middle part of Taizi formation, other layers also widely developed mixed sedimentary rocks, indicating continued input of terrigenous clastic rocks. (2) Sedimentary structure types are rich, and identified eight kinds of physical causes structure, two kinds of chemical genesis structure, four kinds of biogenic structures and two kinds of special causes of structure, structures in shallow water carbonate platform environment include Cross-bedding, tidal bedding, parallel bedding, ripple, trough, stromatolite structure, salt pseudomorphic rock, siliceous concretion, tent structure and mud cake; structures in deep-water turbidite sequence include Graded bedding, parallel bedding, horizontal bedding and erosion bottom surface. (3) Based on characteristics of rocks and types of sedimentary structures, the evolution of sedimentary environment is from stable-relief shallow water carbonate platform characterized by tidal flat environments of carbonate meter-scale sequence (thickness is 1511.95m) to deep-water basin environment characterized by turbidite rock sequence (thickness is 454.14m); The input of detrital material inhibit the production of carbonate, against the rapid formation of beaches and other barrier reef, maintaining the stability of the platform sedimentary; Stromatolite structure generally form biological reef mound, its combination of lithology is siliceous dolomite, crystalline dolostone etc., which is different from the typical carbonate reef, beach facies assemblages, and compared with the salient features of the type of carbonate platform, Shennongjia group is deposited in distal steepened carbonate ramp. (4) Combined with sedimentary characteristics of Shennongding profile, Shennongjia group is divided into six types of facies, Luanshigou formation deposited in intertidal zone, tidal zone and the shallow ramp facies which called rings tidal flat deposits, and may divided into 4 kinds of subfacies, six kinds of microfacies; Dawokeng formation is mainly between intertidal and shallow ramp phase deposition, can be divided into two kinds of subfacies, eight kinds of microfacies; the lower part of Kuangshishan formation is in deep ramp facies, sub-continental shelf subfacies and black shale microfacies, the upper part is characterized by shallow ramp and inter-tidal zone sedimentary facies and is divided into three kinds of subfacies, four kinds of microfacies; Nodular limestone in the lower part of Taizi formation, belong to deep ramp facies and shelf subfacies, the upper is the basinal turbidites subfacies and distal turbidites microfacies sequence. (5) Based on sedimentary evolutionary sequences, and drawing sea level change curve. Sea level of Luanshigou formation is frequent lift, and decline to a maximum regressive period to the top; Dawokeng formation is rising, and mainly in shallow ramp sediments; Kuangshishan formation continued to rise in early period, and frequently in intertidal-shallow ramp later; sea level of Taizi formation is rising for early stage and the in deep-water basin facies for long-term.To sum up, the middle part of Shennongjia group subject to rising sea level in the process, the main evolution is from long-term shallow water of carbonate ramp to short-term deep-water basin of carbonate ramp, and thus this paper establish the distal steepened carbonate ramp depositional model.